The rational development of new antineoplastic agents directed against tubulin, a protein critical for cell division, requires greater understanding of the interaction between the polypeptide subunits of tubulin, its two tightly bound guanine nucleotides, and microtubule-associated proteins. Copolymerization of tubulin GDP and tubulin was studied in further detail, to determine the relative efficiencies with which the two species entered elongating microtubules and to define the minimum concentration of tubulin GTP required to initiate microtubule assembly. The effects of nucleotides on the stability of microtubules continued to be examined, as were conditions to optimize the separation of alpha--tubulin and beta-tubulin on a preparative scale. The purification of a microtubule-associated protein which causes the formation of microtubule bundles continued to progress. Another microtubule-associated protein, which specifically degrades GDP to GMP, was observed and its purification initiated.